I. Field of the Invention
The present invention relates to a device and a process for machining rapidly and with high current densities by electroerosion.
II. Description of the Relevant Art
It is known that, as compared with other more conventional methods, machining by EDM is a very precise but rather slow process. Thus it is of interest to improve the yield of the process, even if this means sacrificing a little precision, which is of little consequence in certain cases, such as in a blank for example.
The main reason for the slowness of EDM machining at present is the fragility of the machining dielectric liquid trapped in the space separating the tool electrode and the work to be machined, called the machining slot or "gap", where it is carbonized, vaporized, ionized, etc., by reason of the temperature rise caused by the sparks which strike between these electrodes, and contaminated by the waste material due to the erosion. Since this "gap" is narrow, generally of the order of 0.005 mm to 0.1 mm, it is difficult to eliminate from it the waste material due to the degradation of the dielectric and to erosion and to renew the latter.
The dielectric liquid most often consists of hydrocarbons and/or water which may contain various additives. The tools most commonly used are either a fine metal wire (with a diameter of some tenths of a millimeter) fed continuously along the work to be machined, or for a sinking electrode.
There are different methods of injecting the liquid into the gap for accelerating the renewal of the dielectric. Although effective to a certain degree, all of these methods find an obstacle in the narrowness of the gap, which prevents a rapid circulation of the liquid which it contains. The destruction of the dielectric and the deterioration of the machining conditions which it brings with it are obviously less rapid during so-called "open geometry" machinings where some renewal of the dielectric is possible than when it is necessary to operate "in closed geometry".
Moreover, in the latter case the gases resulting from the electroerosion and the decomposition of the dielectric are then trapped in the gap and cause the appearance of surface defects. It is mainly for the purpose of avoiding or retarding this decomposition of the dielectric that it has become customary at the start of the machining to limit the intensity of the machining current and consequently the amount of material removed per unit time.
When, despite this precaution, the dielectric starts to degrade during the machining, the known countermeasures range from a temporary intensification of the forced injection of liquid into the gap, due to a device such as that described in British Pat. No. 2,074,074 for example, to an enlargement of the latter or to a decrease in the mean intensity per unit surface of the tool electrode (or density) of the machining current, or to the establishment of a pulsation motion of the tool electrode or else to an increase of the period of rest between successive pulses. Except for the first of these, they all result in reducing the mean current density. The first measure is often ineffective or inapplicable in most of the closed geometries because the flux of the dielectric is non-homogeneous, and the others slow down or interrupt the machining, which reduces its yield. Until the present, it was traditionally necessary to limit oneself to moderate machining current densities, not in excess of 10 amp./cm.sup.2, even when certain countermeasures were adopted, so as to avoid having to lock these in solidly, thus destabilizing the machining. This is why it was customary to machine at slow speed.